It is specified at the outset that the mass flowmeter under discussion comprises, among other items, at least one oscillation driver "assigned" to the Coriolis tube and at least one detector "assigned" to the Coriolis tube. As a rule, the oscillation driver or oscillation drivers (or in any case a part of the oscillation driver or oscillation drivers), and the detector or detectors (or in any case a part of the detector or detectors), are connected to the Coriolis tube. Since this is not necessary, however, the expression "assigned" is used instead of the expression "connected".
In the case of mass flowmeters that operate according to the Coriolis principle, one makes a basic distinction between those in which, on the one hand, the Coriolis tube is at least essentially straight and those in which, on the other hand, the Coriolis tube is loop-shaped. In addition, one distinguishes in the case of mass flowmeters like the one under discussion between those that have, on the one hand, only one Coriolis tube and those that have, on the other hand, two Coriolis tubes. In the constructions with two Coriolis tubes, they can lie in series or in parallel with each other, in a hydrodynamic sense.
In recent times, mass flowmeters with only one essentially straight Coriolis tube have found increasing acceptance. Mass flowmeters with straight Coriolis tubes are mechanically simple to construct and therefore relatively inexpensive to produce. The inner surfaces of their Coriolis tubes are easy to work on (e.g., to polish), they show a small pressure drop, and they are self-emptying.
Despite all their advantages, mass flowmeters with only one straight Coriolis tube are problematic from a number of aspects.
First, because of the thermally-caused expansions and/or stresses associated with a straight Coriolis tube, the accuracy of its measurements depends on the temperature of the flowing medium. In the extreme case, the thermally-caused stresses can even lead to mechanical damage, specifically to stress cracks in the Coriolis tube. This problem is discussed for example in DE 41 24 295 A1 and DE 196 01 342 A1.
Furthermore, the coupling of external perturbations into a mass flowmeter with only one Coriolis tube is stronger than in the case of mass flowmeters in which two parallel Coriolis tubes are operated in the manner of a tuning fork, because in the case of mass flowmeters with only one Coriolis tube, the center of gravity of the mass flowmeter is not fixed. DE 44 23 168 A1 and DE 196 932 500 A1 deal with minimization of the coupling in of external perturbations, i.e., vibrations in the surrounding tubing system.
Finally, DE 197 32 605 A1 deals with the difficulty that arises in measuring the extremely small phase differences between the two signals delivered by the detectors, which are proportional to the mass flow, in the case of a mass flowmeter with a straight Coriolis tube. The usual industrial requirements for accuracy of the mass flowmeter demand a resolution of phase differences of the order of 1.times.10.sup.-5 degrees.
Over and above the problems mentioned above, for which solutions have been approached already in the quoted publications, there is a further problem in mass flowmeters with a straight Coriolis tube, in that the total length of the mass flowmeter is always large in comparison with the nominal length of the Coriolis tube. The large length of the mass flowmeter, relative to the nominal length of the Coriolis tube, results from the necessity, at a given Coriolis tube diameter, to provide a free length of the Coriolis tube over which the Coriolis tube can oscillate with the necessary amplitude. If the Coriolis tube is too stiff, oscillation of the Coriolis tube can not take place with sufficient amplitude to guarantee a reasonable measurement accuracy. The resulting large total length of a mass flowmeter with a straight Coriolis tube is obviously not desirable, since the space available for installation in, for example, the chemical industry and the food preparation industry, where these mass flowmeters are often used, is limited.